1. Field of the Invention
The present invention relates to a process for producing a perfluorocarboxylic acid fluoride, and particularly to a process for producing a perfluorocarboxylic acid fluoride in a high yield, as compared with conventional processes.
2. Description of the Prior Art
Hitherto, it is known that a perfluorocarboxylic acid fluoride is obtained by oligomerization of hexafluoropropylene oxide (hereinafter referred to as HFPO) in various catalyst/aprotic polar solvent systems. Particularly, as for perfluoro (2-propoxy) propanoic acid fluoride (HFPO dimer), many production processes are disclosed, for example, in Japanese Pre-examination Patent Publication (KOKAI) Nos. 57-45132, 62-175437 and 62-195345. According to these known processes, the HFPO dimer is obtained in a yield of 80% or more.
However, when it is tried to obtain a perfluorocarboxylic acid fluoride with a polymerization degree of three or more (i.e., the HFPO trimer or a further higher oligomer), there occur various problems in which, for example, a mixture of perfluorocarboxylic acid fluorides having a broad molecular weight distribution is produced.
For example, according to Japanese Pre-examination Patent Publication (KOKAI) No. 62-195345, it is possible to obtain the trimer of hexafluoropropylene oxide (an HFPO trimer) by effecting oligomerization of HFPO using cesium fluoride as a catalyst, tetraglyme as an aprotic polar solvent and water as a chain transfer agent. By this method, however, it is necessary to set a reaction temperature as low as 0.degree. to 10.degree. C., and a very long reaction time is required. Furthermore, the resultant product has a broad molecular weight distribution, and the HFPO trimer intended does not exceed 40% in the product obtained. In addition, the product may have a wide batch-to-batch variation in content of the trimer. Further, since perfluoropropanoic acid fluoride, that is, an HFPO isomer is produced in a large quantity, much time is spent in treatment for removing the isomer. These problems cause a large increase in production cost.